Cryogenic pumps operated with a two-stage refrigerators are becoming increasingly prevalent due to their comparatively high pumping capacity. The first stage of the refrigerators in such pumps is held at about 80 K and carries pump surfaces in the form of baffles that serve the purpose of condensing water vapor and gases having similar boiling temperatures. These baffles also serve to protect the surfaces of the second stage of the pump against direct irradiation. Gases having comparatively lower boiling temperatures (for example, argon) and light gases (such as hydrogen and helium) are to agglomerate at the surfaces of the second stage of the pump, the temperature of which is approximately 20 K. Hydrogen and helium can be retained on these surfaces by adsorption on these surfaces only if they include activated charcoal or similar adsorption materials. The surfaces of the second stage of a cryogenic pump are therefore designed such that gases proceeding through the baffles initially "see" only those surfaces that serve as condensation surfaces for argon and similar gases. The surfaces covered with adsorption material are shielded, and can be only indirectly reached by the lighter gases. It is therefore possible to filter the condensible gases out before they reach the adsorption surfaces, so that the adsorption material is not unnecessarily loaded with condensible gases. The lighter and thus more mobile gases can then more readily reach, and agglomerate at, the adsorption surfaces.
Many attempts have been made to design the pump surfaces of the second stage of the refrigerators of such cryogenic pumps. Known configurations of such designs can be divided into two groups. In the first group, the pump surfaces are composed of disc-shaped, annularly-shaped, or conically- shaped plates, and have a structure that is dynamically balanced overall (e.g., see European Pat. application Nos. 128 323, 134 942, and 185 702, as well as German Pat. application Nos. 28 21 278, 29 12 856, and 30 38 418). These designs require baffles that, like the pump surfaces, must be constructed in a dynamically balanced configuration.
In the second group, the pump surfaces are composed of a plurality of essentially planar sheet metal sections that are joined together to form a parallelepipedal or cuboid structure (e.g., see European Pat. application No. 196 281 and German Pat. application No. 26 20 880). With designs incorporating pump surface configurations of this type, baffles that are composed of a plurality of metal strips are arranged parallel to one another.
Compared to the pump surfaces of the second group, the pump surfaces of the first group are disadvantageous in that they must be more carefully manufactured and assembled due to their dynamically balanced structure, particularly with respect to equipping pumps of various sizes with such pump surfaces.
Pumps employing pump surfaces of the second group are frequently used in systems involving sputtering processes, which generate comparatively large quantities of condensible gases (particularly argon) and of adsorbable gases (particularly hydrogen). In such pumps, the pumping capacity for these gases is dependent on the conductance of the entrance baffle, but is particularly dependent on the surface that is presented to the respective gas as a entry surface on the inside of the pump. For argon, this "entry surface" is the outer surface of the pump surface configuration. For hydrogen, the entry surface is established by the gaps and openings on the outside surface of the pump surface configuration. Hydrogen can penetrate these gaps and openings to enter into shielded regions having a coating of activated charcoal, upon which the hydrogen agglomerates.
In pumps having pump surfaces of the second type, the planar sheet metal sections are formed as laterally extending side plates. These side plates have outside surfaces which serve the purpose of agglomerating condensible gases. In such pumps, the pumping capacity for argon is therefore dependent on the size of the outside surfaces. Lighter gases such as hydrogen can penetrate to the surfaces covered with adsorption material only from below, or through the end faces, of the pump surface configurations. The pumping capacity of such pumps for hydrogen is therefore dependent on the size of these entry surfaces.
In known pumps having pump surfaces of essentially parallelepipedal or cuboid structure, the two entry surfaces compete with one another to a certain extent. That is, when the surface intended for the agglomeration of argon (i.e., the outside surface of the plates) is enlarged, the entry surface for light gases is reduced in size, thus incurring an associated reduction in the pumping capacity for light gases. The converse is also true, in that when the surfaces through which lighter gases can proceed to the surfaces covered with adsorption material are enlarged, the size of the outer surface is necessarily reduced, thus reducing the pumping capacity for condensing gases.
It can thus be seen that the need exists for a cryogenic pump of the type operated with a two-stage refrigerator, wherein the pump surfaces of the second stage have both an improved pumping capacity for condensible gases as well as an improved pumping capacity for light gases. Moreover, the pump surfaces of the second stage should be able to be manufactured and assembled simply and cost effectively, regardless of the type of pump to which they are to be applied.